(1) Field of the Invention
The present invention relates to the use of shape memory alloys in the construction of devices, which are designed to disengage two components on being heated to a pre-determined temperature. A particular application for the device is to a munitions casing in order to help avoid or at least to mitigate an explosive reaction when such munitions are inadvertently exposed to fire or some other source of heat.
(2) Description of the Art
The present invention is concerned particularly with the use of shape memory alloys (SMAs) as providing means for mitigating against the violent explosive reaction of a munition when it is heated to the ignition temperature of the energetic material. The most extreme condition occurs when the rate of heating is very slow, the so-called “slow cook-off” condition. Under these circumstances, the whole munition reaches an almost uniform temperature so that the casing surrounding the energetic material is unlikely to lose very much strength before the point at which the energetic material finally ignites. At this point there is a rapid pressure build-up and a high order explosion or even a detonation occurs. Faster heating, which occurs for example when the munition is exposed to a fuel fire (a so-called “fast cook-off” condition) is less hazardous and easier to counter. In this situation, because the flow of heat is from the outside of the munition to the inside, the casing will reach a higher temperature than the energetic material and so will weaken before the energetic material ignites. It is possible to enhance this effect by choice of case materials and by the use of thermal insulation (which is usually needed anyway) between the case and the energetic material. Although the present invention is concerned with mitigating both fast and slow cook-off, the emphasis is on the latter because of the lack of alternative measures for meeting this situation.
There have been a number of disasters over the last 40 years, involving ships, magazines and weapon storage depots in which much loss of life and military equipment has been incurred. Alarmingly many of them have occurred during peace time, and, of those that have occurred in wartime, many have not been the result of enemy action.
Slow cook-off events have typically occurred where there is a fire in a compartment next to a magazine, which burns for many hours with the result that the magazine heats up slowly and all the explosive stores within it increase in temperature very slowly and uniformly. Therefore, when the first particle of energetic material reaches its spontaneous ignition temperature (T of I), probably in the range 125° C. to 200° C., the remainder is also on the verge of igniting. Furthermore, at that temperature the munition casings would retain nearly all of their strength, particularly if they were made of steel. The result can be a high order explosion that can, for example, destroy a ship. Two famous examples of disasters initiated by fires are HMS Sheffield in the Falklands War and the USS Forrestal in the Vietnam War, both of which resulted in large casualties and loss of platforms and systems and munitions.
As a result of these and other incidents, the subject of Insensitive Munitions (IM) has become an important one in the design, procurement, storage and deployment of any weapons system that employs propellants or explosives, that is most weapons. There is now a general requirement to design main charges, booster charges, explosive trains, rocket motors and gun propellant charges such that when exposed to a disruptive threat they respond as benignly as possible. Therefore, ideally they should give rise to a burning reaction, rather than a high order explosive event or a detonation. In this way it is hoped to avoid the generation of a shockwave or of damaging fragments that would adversely affect other weapons stored in the proximity. By so doing, the hope is that fratricidal events or “chain reactions” can be avoided.
One way to achieve such IM status is to develop propellants and explosives that are relatively insensitive to shock and fragment attack and much work has been carried out on this over the last 25 years, with new generations of energetic materials emerging, albeit slowly.
Another approach is to design the hardware items, i.e. rocket motor or warhead casing, so that when they are attacked they break open readily and do not allow a rapid pressure build-up that might lead to a detonation or high order explosive event. To some extent, it is difficult to reconcile this requirement with the need to withstand rough handling. Nevertheless some satisfactory compromise solutions have been achieved.
There are several standard IM tests, of which three of the most commonly used are:                Bullet or fragment impact        Fuel fire (so-called fast cook-off)        Slow cook-off (SCO)        
These tests are designed to replicate the common threats that may cause premature, unwanted, detonation of munitions. Methods have been devised for combating the first two of these threats, but mitigating against slow cook-off has remained an intractable problem.
Previously a number of methods have been suggested for attempting to mitigate against premature detonation of munitions under slow cook-off conditions. These have included:                1. The use of line cutting charges on the outside surface of the case, and pointing inwards. Used in association with an appropriate sensor, it can be arranged for such a charge to cut a slit in the case just before the propellant ignites.        2. Thermite blocks have also been used to achieve a similar result by burning a hole in the case.        3. Low melting alloys or polymer compositions have been considered as a means of greatly reducing the strength of a joint when subject to heat.None of these methods has proved particularly successful whether applied to rocket motor cases or to other types of munition. The first two methods are considered as active mitigation methods, which involve the use of additional energetic materials on the body of the weapon, which can introduce a further set of hazards making them an unattractive solution. The third method is referred to as passive mitigation. However, the problem encountered with this type of passive mitigation, using low melting materials, is trying to achieve sufficient strength under normal firing conditions. At the same time it is necessary to ensure that most of the strength has been lost at the lowest possible propellant ignition temperature. For a double base propellant this temperature can be as low as 125° C. An alternative method, by which a low melting point material is used as a fusible plug, is inadequate because it cannot be used to create a large enough aperture for the gaseous products from the propellant or explosive to vent sufficiently quickly.        
Shape memory alloys are metal alloys that undergo large dimensional changes when heated or cooled through a particular transition temperature range. Shape memory alloys exhibit two distinct crystal structures or phases below and above the transition and the mechanical properties of the alloy are different in the two phases. Therefore, upon heating or cooling the alloy, a transition temperature range is reached over which range the crystal phase changes and the alloy will adopt the properties of the new crystal phase. In general, the “memory” is imparted to the SMA by deforming it, usually in the lower temperature state. Therefore a ring which is intended to expand on heating through its transition temperature range would previously have its memory imparted at a lower temperature by compressing it radially. Whereas, a ring intended to shrink on heating would have the memory imparted by stretching. An SMA material is said to exhibit one way memory if the shape change achieved by plastic deformation at a lower temperature is annulled on heating and the deformed shape is not restored on subsequent cooling. By contrast SMA materials which can be made to alternate between a low temperature shape and a high temperature shape throughout a number of heating and cooling cycles are said to exhibit the two-way shape memory. Both types of shape recoveries are possible in most of the SMAs. However the extent of reversible shape recovery associated with two-way shape memory in any SMA is usually less than that associated with one-way memory. In general, though, unlike low melting point metal alloys, which are mechanically weak, SMAs have mechanical properties that are comparable with those of engineering materials such as light alloys and steels and are therefore ideally suited to high stress and strain applications. The transition temperature for the shape change can be selected by the appropriate choice of composition of the SMA.
The one way recovery strain achievable is in the range 2% to 6% in Ti—Ni based SMAs and in the range 1% to 4% in Cu—Al based SMAs. In general, the highest recovery strains are achievable in rings or tubes to which the memory is imparted by stretching in a radial direction and which then shrink to their original dimensions on heating. In the reverse mode, where the memory is imparted by compression and the component expands on heating, the effect is somewhat smaller, but nevertheless large enough to be usable.
A tube manufactured from a shape memory alloy which is designed to expand radially upon heating will usually contract in length at the same time, as the overall volume of the shape memory alloy remains substantially constant. Likewise, if the tube is designed to contract radially, this will lead to a concomitant expansion along the axis. For the purposes of the current invention, it is also significant that many shape memory alloys will generate high recovery strains on activation, even when their movement is opposed by large resistive forces.
Such tubes can be manufactured by machining from rod, forging or extrusion, alternatively; for large diameter tubes it may be more convenient to select SMA alloy sheets of appropriate thickness, wrap them around suitable mandrels to achieve cylindrical shapes and weld the joints to produce SMA tubes. In the latter case there may be some loss of SMA function at the weld interface, but the remaining SMA will give the required expansion or contraction on heating.
U.S. Pat. No. 6,321,656 discloses the use of shape memory alloys to mitigate against slow cook-off in relation to rocket motors. The patent describes three embodiments of the invention as applied to a rocket motor case, which is in two sections. A first section has a small number of prongs each with a small lug at its tip and the second section has an equal number of recesses for location of the lugs. When the two sections are brought together in an end to end manner the lugs engage with the respective recesses by virtue of the prongs on the first section being biased so as to cause each associated lug to lock with its respective recess in the second section. In a first embodiment of the invention, a shape memory alloy ring, which is of an alloy composition such that upon heating it will contract, is located tightly around the prongs. Upon heating, in a thermal hazard incident, the shape memory alloy ring contracts, pushing the prongs inwards and therefore causing the lugs to move out of their respective recesses allowing the two sections of the motor case to disengage and so to vent any built up pressure. In a second embodiment, the shape memory alloy ring is placed on the inside of the prongs on the first section, and is expanded so as to force the prongs into engagement with their corresponding recesses. On heating the ring retracts to its annealed size thereby allowing the prongs on the inner section to move inwards away from engagement with the respective recesses in the outer section. In the third embodiment, the first section is slightly modified to allow the location of two shape memory alloy rings, one around the outside and one on the inside of the pronged section, thus providing the combined effects of the first and second embodiments, such that upon heating both rings contract inwards, to give the same overall effect.
However, the arrangement shown in the US patent suffers from the disadvantages that once the ring or rings have been put into position, they cannot be easily removed without heating the device. It is common practice for munitions to be regularly serviced and monitored during their service life and so a non-reversible system such as this would not be an ideal solution. Another disadvantage is that the pronged section produces an internal projection into the volume where the propellant is located. This results in difficulties for loading the propellant when in cartridge form into the rocket casing and means that the propellant would most likely have to be melt cast. A further disadvantage of the arrangement shown in this US patent is that the shape memory alloy has to be heat treated to enable the connection means to be installed. In addition, as the whole of the axial load arising from the pressurisation of the case has to be carried through the prongs and lugs, the arrangement is structurally inefficient. Finally, the shape memory alloy ring in this arrangement is not an integral part of the connection system, thus adding to the complexity of the arrangement and hence the cost of manufacture.